Spot diagnosis of skin disorders


This woman presented with a history of an irritating rash that had been present for a few months. It affected mainly her wrists, abdomen, back and groin. Inspection of the lesions at the wrist immediately suggested the diagnosis of lichen planus. The lesions were flat-topped, purplish or violaceous papules. The linear streak of lesions that had developed along the line of a recent scratch are known as the Koebner phenomenon. As well as these signs, fine white streaks (Wickham's striae) could be seen running across the surface of the papules.


This 'herald patch' should be sought in a case where a diagnosis of pityriasis rosea is suspected. It appears as a pink oval patch with a collarette of scale towards the periphery of the lesion but not at its margin. It might measure between 3-6cm in size. Within a few days further, smaller symmetrically placed lesions develop usually on the neck, chest, upper abdomen, back, upper arms and thighs. The patient remains well and the rash is not usually itchy. Presenting alone, the herald patch might be confused with an annular patch such as discoid eczema or ringworm; however, when the rest of the rash appears, the true cause is certain.


Scabies may be suspected in any patient who presents with a very itchy, non-specific rash, especially if family or other contacts have similar lesions. The search for burrows from which the scabies mite can be extracted will confirm the diagnosis when the mite is viewed under the microscope. In men with scabies, it is common to find papules on the penis, which are pathognomonic of scabies in an itching patient. These nodules might persist even when the condition has been adequately treated.


Molluscum contagiosum lesions are flesh-coloured, dome-shaped papules which develop a central depression. It is an infection caused by a DNA virus of the pox group. The most important diagnostic sign is the umbilication. Multiple lesions occur and might be seen on any part of the body, particularly on the face and neck. Patients with eczema are particularly prone to molluscum contagiosum and might have widespread lesions. In adults, lesions on the lower abdomen or genitalia would suggest they have been acquired during sexual activity and referral to exclude other STIs should be considered.


This patient presented after the initial vesiculobulbous rash had subsided. Hyperpigmentation always follows resolution of the rash. The rash had a bizarre network of geometric streaks. After questioning the patient described that the rash followed a sunny day when she had been working among the parsnips and celery on her allotment. Both these plants contain a chemical that reacts to ultraviolet light, causing blistering and pigmentation.


This patient complained of dandruff, however, the examination suggested psoriasis when discrete red plaques covered in a silvery white scale were found on the scalp and along the hairline. If the scale is scraped away small bleeding points are revealed. Such plaques may occur anywhere on the skin, but are commonly seen on the scalp, as well as the extensor surfaces of the elbows, knees and sacrum.

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